1. Field of the Invention
The present invention relates to a fuel assembly acoustic system tester for identifying leaking nuclear fuel assemblies, and, more particularly, to a system responsive to vibrations emitted from a leaking fuel assembly during removal from a nuclear reactor core such as during refueling of the nuclear reactor.
2. Description of the Prior Art
As disclosed in my U.S. Pat. No. 4,039,376, it is known in the art to inspect nuclear fuel rods and assemblies of fuel rods to detect and locate defective rods. As explained in the patent, the large nuclear reactors utilized for power generation consist of an array of a large number of fuel rods containing nuclear fuel. Each rod comprises a metal tube which may be from 8 to 15 feet long and approximately 1/2 inch in diameter, and which contains a stack of cylindrical fuel pellets of suitable fissionable material such as uranium oxide. The upper end of the tube is empty of fuel pellets and forms a plenum for a gas or other fluid under substantial pressure which fills the top of the rod and also the small clearance space which is provided around the pellets to allow for expansion or swelling. The fuel rods are supported in parallel groups as fuel assemblies which may typically contain from 49 to as many as 300 fuel rods, and the complete nuclear reactor is made up of a large number of these fuel assemblies arranged in a suitable configuration.
The metal tubes of the fuel rods constitute the primary containment boundary for the radioactive nuclear fuel, and inspection to verify the integrity of the rods is of primary importance. In the manufacture of the fuel rods, the tubing itself and the end cap welds are carefully inspected and helium leak tests of the completed rods are also performed. Since a nuclear reactor may contain as many as 40,000 fuel rods, however, it will be apparent that a significant probability exists that some number of defective tubes will be present even with a highly effective manufacturing quality control program. Furthermore, even initially good fuel rods may develop cracks, pinholes or other defects in service and such defective rods must be detected.
The reactor is usually shut down about once a year for refueling and at this time, as well as during initial installation, the fuel rods must be inspected to detect any defective rods that may be leaking fission products. The reactor and the fuel assemblies are immersed in a pool of water during the refueling operations and during removal of the fuel assemblies for replacement or inspection. In the standard method of inspection which has been used heretofore, the flow of water through each fuel assembly to be inspected is blocked so as to allow the fuel rods to heat up, which causes expulsion of fission products into the water through any defect that may exist in the rods. The water is then checked to detect the presence of radioactivity in the water, indicating that such leakage of fission products has occurred and that a defective rod is present in the assembly being tested. This system requires that the water be pumped to a sampling station and repeatedly analyzed or checked for radioactivity and is a very slow procedure requiring as much as an hour for each fuel assembly. Complete inspection of all fuel rods in a large power reactor is, therefore, very consuming and may require as much as a week during which the reactor is out of service. This represents a very high cost to the operator of the reactor and a more rapid but simple and reliable inspection method is needed.
It is also known in the art to provide a method and apparatus for inspecting nuclear fuel rods in a much more rapid and inexpensive but highly reliable manner. This method consists essentially in causing a change in the relative pressures of the fluid within the fuel rods and the water in which they are immersed such that the internal pressure within the rods becomes higher than the external water pressure. If any defect such as a crack or pinhole exists in any of the fuel rods, the result is the expulsion of a bubble or bubbles of fluid or other material from the interior of the fuel rod into the liquid. Sensing means are provided to detect the emission of such bubbles to provide a reliable and instantaneous indication of the presence of the defect and also of the defect. This provides a very sensitive and reliable method of inspection, but it is adsorbed by the fuel pellets, particularly at the top of the rod, and in some cases such trapped fluid is not expelled and defects in the upper part of the rod may not be detected.
As disclosed in my U.S. Pat. No. 4,039,376, an assembly of fuel rods is first tested by changing the relative pressure that is produced such that fluid within the rod is expelled into the surrounding water through any defect that may exist and the emission of bubbles or other material so expelled is detected. The change in relative pressure is accomplished by blocking the flow of cooling liquid so that the fluid in the rod is heated to increase its pressure, or pressurizing the fuel assembly and then releasing the external pressure to reduce the pressure relative to the internal pressure of the fluid within the rod, or by raising the fuel rod assembly in the liquid in which it is immersed to a higher level where the external pressure is lower. The fuel rods or the complete assembly of rods are vibrated by means of any suitable type of vibrator, and this vibration effectively drives out or expels fluid trapped in the upper part of the fuel rod through any defects which may exist there even though such fluid could not have been expelled by the change in pressure. This additional step, therefore, provides an effective and sensitive means for detecting the existence of defects, especially in the upper part of the rod, which were not necessarily detected by the previous method.
These known fuel rod testing procedures notwithstanding improvements that have been made still require considerable time and additional equipment for the testing procedure. More recently, the task of finding and removing failed fuel assemblies from the reactor has become a top priority for nuclear utilities. Fission products leaking from failed fuel assemblies can cause many conditions that increase operating costs. These conditions include (1) high radiation readings in the primary coolant system; (2) increased volume of liquid radioactive waste; (3) increased volume of solid radioactive waste due to more frequent demineralizer bed replacement; (4) increased costs for disposal of spent fuel assemblies due to special handling and additional decontamination; and (5) increased exposure to personnel. Currently fuel assemblies are tested using fuel "sipping" and/or Ultrasonic Testing (UT) techniques. Fuel sipping detects the release of fission product gases from a failed rod(s) within a fuel assembly. Sipping is performed either in the mast of the refueling machine or in a closed container in the spent fuel pool. UT detects the presence of water inside the fuel cladding of a particular fuel rod(s) within an assembly. UT generally is performed in the spent fuel pool. Sipping requires 2 to 5 minutes per assembly to sample and test for fission products plus an additional 5 to 60 minutes to purge the system whenever a leaking assembly is detected. UT requires 10-15 minutes per assembly for handling and scanning. Although the time requirements for sipping and UT for a single assembly do not seem significant, the time for testing an entire core is approximately two days. With increasing pressure to decrease outage durations, the nuclear industry would benefit immensely from a leak detection system that would reduce the time required to locate leaking assemblies and fuel rods. An effective means of identifying potential leaking assemblies as they are removed from the core, and one that is not dependent upon fission product detection would eliminate the time required for sampling and purging. It would also reduce substantially the time required for UT by pre-screening the assemblies thereby reducing the number to be ultrasonically scanned to a small fraction of the total.
Accordingly, it is an object of the present invention to provide a fuel assembly acoustic system tester for detecting vibrations emitted from leaking fuel assemblies as they are removed from the core assembly of a nuclear reactor.
It is a further object of the present invention to detect vibrations emitted from leaking fuel assemblies during upward motions of the fuel assembly being removed from a core of a nuclear reactor together with decreases to the external hydraulic pressure on the fuel rods so as to increase the pressure differential across the fuel rod cladding whereby if a leak is present the differential pressure causes the expulsion of internal fuel rod fluid, gas or liquid, through the leak.
It is another object of the present invention to derive an electrical signal within a selected response frequency to establish essentially only mechanical vibrations corresponding to only the passage of a fluid medium through porus sidewall defects in the fuel rod of the fuel assembly during removal from a nuclear reactor core.